Thermally conductive interface

ABSTRACT

A thermally conductive material is provided as a mixture of a silicone, a ceramic powder, and a curing catalyst. The material may be pre-formed into a pad and each side of the film protected with removable release layers. Each side of the film may also include a coating of an adhesive material that aids in coupling the interface film with a surface. The material may alternatively be produced in a screen-printable paste. As such, a layer of the paste may be screen-printed on a surface as complete sheet form or as a patterned film by using a stencil patterned screen mesh. The interface material is sandwiched between a printed circuit board and a heat sink to form the circuit board assembly. In a multi-step press process, the assembly is cured and a laminate formed. The assembly process may also include a priming function that prepares metal surfaces of the circuit board and heat sink for receiving the interface material.

BACKGROUND

1. Field

This disclosure relates to thermal interfacing in electronic devices andmore specifically relates to using an interface material to adjoin aprinted circuit board with a heat sink.

2. Description of Related Art

As electronic components become smaller and more powerful, their heatdissipation requirements also rise dramatically. When an aluminum heatsink is attached to a printed circuit board, an interface must be usedthat ensure a proper thermal connection. The interface may also serve asan electric insulator.

Historically, both thermal grease and pre-formed interface pads havebeen used as interfaces. However, the present state of the art can bedifficult to work-with, more expensive, and/or unreliable. Therefore,there is a need for an advance in the art of preparing thermal interfacematerial and in manufacturing circuit board assemblies using theinterface material.

SUMMARY

This disclosure relates to a thermal interface material and itsapplication in a circuit board assembly. A thermally conductive materialis provided as a mixture of a silicone-based compound, a ceramic powderfor enhancing thermal conductivity, and an organic curing catalyst. Thethermally conductive material may be pre-formed into a film or pad andeach side of the film protected with removable release layers. Each sideof the film may also include a coating of an adhesive material that aidsin coupling the interface film with a metal surface. The thermallyconductive material may alternatively be produced in a screen-printablepaste. As such, a layer of the thermally conductive paste may bescreen-printed on the metal surface. The screen used for printing thepaste on the metal surface can be either without any stencil pattern toallow the entire metal surface to be covered with the paste, or thescreen can be mask patterned suitably to allow the paste applied in thecorresponding pattern form on the metal surface.

The interface material is sandwiched between a printed circuit board anda heat sink to form the circuit board assembly. In a multi-step pressprocess, the assembly is cured and a laminate formed. The multi-stepprocess includes a first pressure treatment applied to the assembly at aroom temperature to increase surface contact and to remove air pockets,a second pressure treatment applied to the assembly at a hightemperature to cure the interface and create a laminate; and a thirdpressure treatment applied to the assembly at a low temperature tocontrollably return the assembly to room temperature. The assemblyprocess may also include a priming function that prepares metal surfacesof the circuit board and heat sink for receiving the interface material.

The interface material may also be used in other applications, such asmulti-layer circuits and to fill vias and channels in a circuit board.Further, the material may serve as an interface between a circuitcomponent and a heat sink or other element.

The foregoing as well as other aspects, advantages, and alternativeswill become apparent to those of ordinary skill in the art by readingthe following detailed description and claims, with reference whereappropriate to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional drawing illustrating a circuit boardassembly;

FIG. 2(a) is a perspective view of an interface film in sheet form;

FIG. 2(b) is a perspective view of an interface film in roll form;

FIG. 3 is a flowchart illustrating a process flow for a process ofmanufacturing a circuit board assembly using pre-manufactured interfacefilm;

FIG. 4 is a flowchart illustrating a process flow for a process ofmanufacturing a circuit board assembly using an interface material in ascreen-printable paste form; and

FIG. 5 is a simplified schematic diagram illustrating application of theinterface material in a multilayer assembly.

DETAILED DESCRIPTION

1. Overview

Referring to the drawings, FIG. 1 is a diagram illustrating anembodiment of a circuit board assembly. In the assembly, a thermallyconductive interface 104 is sandwiched between a circuit board 102 and aheat sink 106. In operation, excess heat generated by components mountedon the circuit board 102 may be shunted through the interface 104 to theheat sink 106.

Generally, the interface 104 is composed of a silicone-based dielectricand ceramic mixture having a low thermal impedance. When cured in ahot-press method, the interface 104 also provides a mechanical bond tohelp secure the assembly. An organic catalyst is included to aid in thecuring process.

The circuit board 102 is a printed (or printable) circuit board (PCB) onwhich electronic components may be mounted and may include anon-conducting substrate layer, such as a fiberglass layer. In addition,the circuit board 102 may include a metal base layer constructed ofcopper, for instance.

The heat sink 106 is a metal element, such as an aluminum, brass orcopper element configured to receive a heat transfer along a planar sideand to release excess heat from its other surfaces. While the heat-sinkmay be configured as a simple rectangular prism, other configurationsthat may add additional surface area may be appropriate.

In manufacture, the interface 104 may be applied either as ascreen-printable mixture or as a pre-formed laminate.

2. Silicone Based Thermally Conductive Material

In the embodiment described above, the thermally conductive interface iscomposed of a mixture with a silicone-based compound as the primaryingredient. The silicone-based compound may, for instance, be a dimethylsilicone and/or methyl-poly silicone oxide. Such compounds may bebeneficial, as they are easily obtainable and are more resistant tothermal shock than other compounds such as an epoxy.

Ceramic particles are included in the interface mixture to providethermally conductive properties. Ceramic particles should be selectedaccording to thermal needs of the specific application. Examples ofceramic particles that may be used include aluminum oxide, aluminumnitride, and boron nitride. Of course, a ceramic compound may beselected based on the required thermal properties of the material.Further, a combination of ceramic compounds may be blended into thesilicone-based material to provide alternative thermal properties. Inanother embodiment, the ceramic particles may be pre-ground to a size ofapproximately 10 nm.

A softness catalyst is included in the film to control a softness of thefilm as it is manufactured. For instance, a nickel-based catalyst may beused. Additionally, a curing catalyst may be added to promote and/orcontrol the curing reaction. The curing catalyst may also serve as ahardener. Examples of the curing catalyst may include a peroxide-basedor a platinum-based catalyst. A thinning agent may be used to controlviscosity of the mixture.

It will be appreciated that other materials may be added to theinterface mixture to provide various functionalities. Likewise,materials listed here may be removed or substituted in some situations.

3. Laminate Layer

In certain embodiments, the thermally conductive interface ispre-manufactured (pre-formed) film (laminate) that can be placed betweena circuit board and a heat sink during assembly.

FIG. 2(a) illustrates an embodiment of such an interface film in aprotected sheet form. As manufactured, a protected interface film 200includes the interface film 204, a first release layer 202 protecting afirst side of the interface film 204 and a second release layer (notshown) protecting a second side of the interface film 204. In oneembodiment, these sheets are sized at approximately 18′×12′×4 mil. Inoperation, the release layers are removed to reveal the bare interfacefilm 204 prior to placing the film within the circuit board assembly.

FIG. 2(b) illustrates an embodiment of an interface film in a protectedroll form. As manufactured a rolled protected interface film 250includes the rolled interface film 256, a first rolled release layer 252protecting a first side of the rolled interface film 256 and a secondrelease layer (not shown) protecting a second side of the rolledinterface film 256. A roll 258 allows a large amount of interface filmto be stored without unduly bending or crimping the film. In operation,a portion of the rolled protected interface film 250 may be unrolled andcut according to manufacturing needs. For purposes of this disclosure,the term “interface film” is defined to include, but is not limited to,sheet form and rolled form.

In certain embodiments, an adhesive is included with the interface filmto promote bonding with the metal surfaces of the heat sink and circuitboard. In one application, the adhesive is added to the surface of bothsides of the interface film prior to application of the release layer.In an alternative application, the adhesive is added to the interfacemixture prior to forming it as a film.

4. Lamination Process

FIG. 3 illustrates a process flow for manufacturing a circuit boardassembly with a pre-manufactured interface film, such as those describeabove. At block 302, the release layers are removed from each side ofthe interface film to expose the film. At block 304, the interface filmis then sandwiched between the circuit board and the heat sink. AlthoughFIG. 3 shows block 302 occurring prior to block 304, in otherembodiments these steps may be executed in an intertwined fashion. Thisintertwined fashion may include removing a first release layer from afirst side of the interface film, then pressing the exposed side againsta planar side of the heat sink. Once the interface film is (e.g.,loosely) attached to the heat sink, the second release layer is removedto expose a second side of the interface film. The circuit board is thenpressed against the second side to form the sandwich assembly shown inFIG. 1. Of course, in another embodiment, the intertwined function maybe reversed so that the interface film is first attached to the circuitboard and then attached to the heat sink. At this point, the surfaces ofthe interface film are pliable and, therefore, allow a high rate ofsurface contact.

Once the sandwich assembly is formed, at block 306, a room-temperaturepressure treatment is applied to the assembly—pressing the heat sinktoward the circuit board. In operation, it is expected that thispressure treatment may be applied using a roller-assembly or any othernumber of mechanisms. The room-temperature pressure treatment works to(i) substantially remove any air-pockets that could reduce thermalconductivity and create ‘hot spots’ in the assembly and (ii) increasesurface contact at the circuit board/film boundary as well as the heatsink/film boundary.

At block 308, a high-temperate pressure treatment is applied to theassembly—again pressing the heat sink toward the circuit board. Thehigh-temperature press is intended to promote curing of the interfacefilm as well as bonding of the interface film to the adjacent metal.Typically, the high-temperature bond may operate at a temperature ofapproximately 330 degrees Fahrenheit and a pressure of approximately 150psi for approximately 20 minutes. Of course these parameters may varyaccording to a number of factors, such as the thickness and compositionof the interface layer and the particular requirements of any curingcatalyst used. In a further embodiment, the high-temperature pressuretreatment includes application of a temperature of at least 320 degreesFahrenheit and pressure of at least 140 psi for at least 20 minutes.

At 310, a low-temperature pressure treatment is applied to theassembly—again pressing the heat sink toward the circuit board.According to the exemplary embodiment, the low-temperature pressuretreatment is applied immediately following the high-temperature pressuretreatment. The low-temperature may be room-temperature or another valueat or below room temperature. In a further embodiment, thelow-temperature is not a fixed temperature, but is a temperature that isreduced over time during the low-temperature pressure treatment.

In the high-temperature pressure treatment, the silicone matrix formscross-links that are hardened/cured. The low-temperature pressuretreatment cools the interface down to room temperature under pressurewithout letting any air trap between the bonded layers. This may reducethe occurrence of delamination of the bonded layers. Once the assemblyis cooled, circuit components may be assembled on the circuit board. Incertain embodiments, the low-temperature pressure treatment is performedat a pressure of approximately 40 psi for approximately 10 minutes.

In a further embodiment, the circuit board has a metal base (such as acopper or aluminum base) that is attached directly to the interfacefilm. Likewise, a planar surface of the heat sink is attached to theother side of the interface film.

Prior to attaching the circuit board and heat sink to the interfacefilm, it may be appropriate to prepare the metal surfaces—thus helpingto ensure better adhesion to the film. The preparation may include, forinstance, degreasing, desmutting, physical roughening and chemicalroughening of the metal surface and then cleaning the surface withalcohol and/or applying a thin coat of a primer material. Thisconditions the surface for better adherence to the material.

In a further embodiment, the primer material may be a chromate primer.(e.g., chromic acid). In another embodiment, anodizing the planarsurface of the heat sink may serve to prepare the surface for bindingwith the interface film.

In an exemplary embodiment, the end result of the lamination process isthat the circuit board assembly becomes a single element—the interfacefilm bonded securely with both the metal bottom of the PCB and theplanar surface of the heat sink. In some cases, excess interfacematerial from an edge of the assembly may be trimmed.

5. Screen-Printing Process

In another exemplary embodiment, the thermally conductive mixture isprovided in a screen-printable paste form. The screen printable form mayprovide a lower cost mechanism for creating a thermally conductiveinterface between the circuit board and the heat sink.

FIG. 3 provides an exemplary process flow for manufacturing the circuitboard assembly with the thermally conductive mixture in screen-printablepaste form. At block 402, the metal surfaces of the circuit board andheat sink are cleaned and primed. As described above, this may includedegreasing, desmutting, physical roughening and chemical roughening ofthe metal surface and then cleaning the surface with alcohol and/orapplying a thin coat of a primer material.

At block 404 an interface layer is screen printed onto one of the metalsurfaces. According to various embodiments, either the metal base of theprinted circuit board or the planar surface of the heat sink receivesthe screen printed layer. The screen printing may be adjusted to applyvarious layer thicknesses and pattern according to manufacturingspecifications. In addition, the screen printing may be patterned toavoid artifacts in the circuit board such as vias and posts, forinstance. The screen patterning technique includes stencil formation ofa pattern on the screen. In a further embodiment, the screen-printingstep is repeated until the interface layer is a desired thickness. Thescreen printing can be performed manually or by a screen printingmachine.

At block 406 the interface layer is then sandwiched between the circuitboard and the heat sink to create the assembly.

Once the sandwich assembly is formed, at block 408, a room-temperaturepressure treatment is applied to the assembly—e.g., pressing the heatsink toward the circuit board. In operation, it is expected that thisroom-temperature pressure treatment may be applied using aroller-assembly or any number of other mechanisms (as may the otherpressure treatments). The room-temperature pressure treatment works to(i) substantially remove any air-pockets that could reduce thermalconductivity and create ‘hot spots’ in the assembly and (ii) increasesurface contact at the circuit board/film boundary as well as the heatsink/film boundary. Of course, the room temperature pressure treatmentmay provide other benefits as well.

At block 410, a high-temperate pressure treatment is applied to theassembly—again pressing the heat sink toward the circuit board. Thehigh-temperature press is intended to promote curing of the interfacefilm as well as bonding of the interface film to the adjacent metal.Typically, the high-temperature bond may operate at a temperature ofapproximately 330 degrees Fahrenheit and a pressure of approximately 150psi for approximately 20 minutes. Of course, these parameters may varyaccording to a number of factors, such as the thickness and compositionof the interface layer and the particular requirements of any curingcatalyst used. In a further embodiment, the high-temperature pressuretreatment includes application of a temperature of at least 320 degreesFahrenheit and pressure of at least 140 psi for at least 20 minutes.

At block 412, a low-temperature pressure treatment is applied to theassembly—again pressing the heat sink toward the circuit board.According to the exemplary embodiment, the low-temperature pressuretreatment is applied immediately following the high-temperature pressuretreatment. Once the assembly is cooled, circuit components may beassembled on the circuit board. For this embodiment, the low-temperaturepressure treatment is accomplished at approximately 40 psi forapproximately for 10 minutes or until the assembly is properly cooled.

6. Alternative Embodiments

Of course, the silicone-based thermal interface as described may beuseful in more applications than those specifically described in theexamples above. For instance, FIG. 5 illustrates a multilayer circuitboard using such a thermal interface. FIG. 5 illustrates a multilayerprinted circuit board with multiple layers 508, 506 that are separatedby a first thermal interface 510. Thermal vias 514, 512 may thermallycouple the first thermal interface 510 with a second thermal interface504, where the second thermal interface 504 is furhter coupled with aheat sink 502.

In the embodiment of FIG. 5, the thermal interface layers 504, 510 maybe either pre-manufactured thermal interface films or screen-printedthermal interface layers. Filling the vias 514, 512 may be accomplishedvia screen-printing, injection, or other mechanical methods. Of course,FIG. 5 is a simplified embodiment. Other embodiments may include agreater number of thermal vias, as well as more circuit board layers.The thermal interface may also be useful to fill thermal channels that,for instance, shunt heat to an edge of the circuit board. The thermalchannels may be filled in a similar fashion as the thermal vias. In adouble-sided circuit board, the thermal interface may also be used toshunt heat to a heat sink.

In yet another embodiment, the thermal interface material may be used tocouple an electronic component directly to a heat sink. For instance,the thermal interface may be used to couple a processor, an electricmotor, or a power source directly with a heat sink.

7. Material Properties

Typical data for a four mil thermal silicone film is provided inTable 1. The results shown in Table 1 are a summary of data obtainedfrom test results performed on a preformed thermally conductive layerconstructed in accordance with an exemplary embodiment. TABLE 1 TestMethod Property Value ASTM D374 Thickness 4 mil. ASTM D412 TensileStrength 500 psi ASTM D2240 Hardness (Shore A) 73 ASTM D412 Elongation250% Continuous Use −40 to 450 Degrees Temperature Fahrenheit ASTM D5470Thermal Conductivity 0.37 W/m-k ASTM D257 Electrical Volume Resistivity10¹²-10¹³ Ohm-cm Bond Strength 15 to 16 PPI Specific Gravity 1.43

Of course, other embodiments may be constructed to achieve alternativeresults. For instance, a thermal film may be manufactured with anythickness ranging from two mil to two-hundred mil. According to apreferred embodiment, it is important to maintain a substantiallyuniform thickness across a manufactured film in order to ensure a strongbond with the circuit board and heat sink.

Thermal conductivity of an interface film is a function of theparticular combination of ceramic powders added to the mixture. Typicalceramic powders that may be used include aluminum oxide, aluminumnitride, and boron nitride. Boron nitride may provide better heatconductivity, although a combination of powder types may also bebeneficial. Of course, these listed ceramic powders are only given byway of example, and other materials may be used. Depending upon theceramic powder(s) selected and the quantity of that powder(s), a thermalconductivity of the film may be increased to approximately 2.5 W/m-k.

8. Conclusion

While a number of exemplary aspects and embodiments have been discussedabove, it will be appreciated that certain modifications, permutations,additions and sub-combinations thereof can be made. It is thereforeintended that the following appended claims and claims hereafterintroduced are interpreted to include all such modifications,permutations, additions and sub-combinations as are within their truespirit and scope.

1. A method of manufacturing a circuit board assembly having improvedheat dissipation functionality comprising: providing a thermallyconductive, electrically non-conducting interface film including: afirst side and a second side wherein the interface includes resilientsilicone, a ceramic powder for enhancing thermal conductivity, anorganic curing catalyst, a first release layer protecting a first side,and a second release layer protecting a second side; sandwiching theinterface between a metal base layer of a printable circuit board and aplanar side of a heat sink to form the assembly; in a mechanism forapplying pressure, applying a first pressure treatment to the assemblyat a room temperature to increase surface contact and to remove airpockets; applying a second pressure treatment to the assembly at a hightemperature to cure the interface and create a laminate; and applying athird pressure treatment to the assembly at a low temperature tocontrollably return the assembly to room temperature.
 2. The method ofclaim 1, wherein the resilient silicone is a dimethyl silicone.
 3. Themethod of claim 1, wherein the ceramic powder includes a ceramicselected from the group consisting of aluminum oxide; aluminum nitride;and boron nitride.
 4. The method of claim 1, wherein the interface filmfurther includes: a first adhesive applied to the first side; and asecond adhesive applied to the second side.
 5. The method of claim 1,wherein the second pressure treatment includes application of atpressure of at least 140 psi at a temperature of at least 320 degreesFahrenheit for at least 20 minutes.
 6. The method of claim 1, furthercomprising: trimming excess interface material from an edge of theassembly.
 7. The method of claim 1, wherein the interface film has apre-laminate thickness of between 2 and 200 mils.
 8. The method of claim1, wherein the interface film has a pre-laminate thickness ofapproximately 4 mils.
 9. A method of manufacturing a circuit boardassembly having improved heat dissipation functionality comprising:screen-printing a thermally conductive, electrically non-conductiveinterface layer onto a first planar surface of one of a printablecircuit board and a heat sink, wherein the interface layer comprises amixture of dimethyl silicone, a ceramic powder for enhancing thermalconductivity, and a curing catalyst; sandwiching the interface layerbetween a the first planar surface and a second planar surface of theremaining one of the printable circuit board and the heat sink to formthe assembly; applying a first pressure treatment to the assembly at aroom temperature to increase surface contact and to remove air pockets;applying a second pressure treatment to the assembly at a hightemperature to cure the interface and create a laminate; and applying athird pressure treatment to the assembly at a low temperature tocontrollably return the assembly to room temperature.
 10. The method ofclaim 9, wherein the second pressure treatment includes application ofat pressure of at least 140 psi at a temperature of at least 320 degreesFahrenheit for at least 20 minutes.
 11. The method of claim 9, whereinthe third pressure treatment includes application of at pressure ofapproximately 40 for at approximately 10 minutes.
 12. The method ofclaim 9, wherein the second pressure treatment includes application ofat pressure of approximately 150 psi at a temperature of atapproximately 330 degrees Fahrenheit for at approximately 20 minutes.13. The method of claim 9, prior to screen-printing the interface layer,cleaning, surface roughening and applying a primer to the planar surfaceof the printable circuit board.
 14. The method of claim 9, by which filmis applied in a continuous form or as a stencil patterned format,further comprising: repeating the screen-printing function apredetermined number of times to achieve a desired interface thickness.15. The method of claim 9, wherein the organic curing catalyst is aperoxide.
 16. The method of claim 9, wherein the ceramic powder includesa ceramic selected from the group consisting of aluminum oxide; aluminumnitride; and boron nitride.
 17. The method of claim 9, furthercomprising: trimming excess interface material from an edge of theassembly.
 18. The method of claim 8, wherein the dimethyl silicone is amethyl-poly silicone oxide.
 19. A thermally conductive interface forinterposing between an aluminum heat sink and a copper base of anelectronic device comprising: a dielectric including a mixture of amethyl-poly silicone oxide, a ceramic powder for enhancing thermalconductivity; and a peroxide curing catalyst, wherein the dielectric isspecially formulated to serve as a thermally conductive electricinsulator; a first adhesive applied to a first side of the interface,the first adhesive being configured to adhere to the aluminum heat sink;a second adhesive applied to a second side of the interface, the secondadhesive configured to adhere to the metal base; a first release layerprotecting the first side; and a second release layer protecting thesecond side.
 20. The interface of claim 19, wherein the interfaceincludes a thermal conductivity of approximately 0.37 W/m-k.